This invention relates to a method and a device for determining leaks of the seal of a valve or of a valve rod and in particular to the determining of such leaks in a process valve.
In accordance with the prior art, the leadthroughs of valve rods, which connect the drive to the valve slide, on engineering valves, i.e. on process valves, are sealed using packing made from a very wide range of materials and designs or using lip seals.
While the valve is being used, wear and/or damage to the packing and seals may occur, leading to leaks. A leak leads to the medium which is controlled by the process valve being able to pass into the environment. This undesirable effect is known as an external leak or a leak to the environment.
Depending on the physical and chemical properties of the medium, on the nature of the application and on the location of use, a continuous leak may be permissible within certain limits. For other applications, leaks of any type must be avoided wherever possible. Particularly when using flammable and explosive media and toxic substances or substances which are difficult to degrade, leaks may entail devastating consequences for the environment or may put people and equipment in danger. Also, when using valuable substances, the financial losses caused by leaks must not be underestimated.
The invention is therefore based on the object of developing a method and a device that can reliably and safely recognize a leak in the region of the valve rods of process valves so that an indication that the valve-rod seal needs to be changed can then be obtained in good time.
With regard to the method, the core idea of the invention is that, to determine the leak, the pressure is determined, if appropriate as a function of time, in a control volume between a first valve-rod seal and a second valve-rod seal, and this pressure is used to calculate the leak flow rate. Compared to a known design of valve, there is now, as it were, a further seal connected downstream of the valve-rod seal. However, this further seal is not acting as a redundant seal as part of a sealing assembly, but rather is used to create a defined control volume between the conventional seal and the second seal, and this volume is monitored in the illustrated manner.
Although a simple redundant design produced by a seal assembly comprising a plurality of seals, as is often used in the prior art, does provide a relatively good sealing action for a limited time, the leak per se cannot be detected. By contrast, the method according to the invention provides a very reliable way of determining the leak and therefore determining the state of the seal.
In an advantageous configuration, the value for the leak flow rate is used to draw conclusions as to whether the seal is sealed, and in this way a seal replacement time is determined.
Furthermore, it is advantageous for the control volume to be discontinuously opened or emptied and then closed again, and for the pressure rise in the control volume then to be measured. In this way, it is possible to reproduce a fixed starting point, from which measurements can always be carried out.
In a further configuration, the pressure of the control volume is limited, and the control volume is emptied when an upper pressure level is reached, until a low closing pressure has been established.
The control volume may in this case be vented or emptied via a switching valve which is actuated by pressure switch means, and a leak flow rate can then be determined as a function of the closing time, the pressure in the control volume and the state equations of the working medium.
Alternatively, the procedure can be such that an upper pressure level and a low closing pressure are recorded by pressure switches or by a pressure sensor, and then a leak flow rate is determined as a function of the closing time, the pressure in the control volume and the state equations of the working medium.
Furthermore, it is advantageous to drive the switching valve using a control programme in such a way that when a test cycle is initiated, the individual steps of the test cycle take place automatically. The test cycle then advantageously includes the following steps:
opening the switching valve and venting the control volume,
closing the control volume and starting the control time,
determining a pressure rise by recording the control volume pressure over time,
evaluating and generating a message or an alarm in the event of predetermined pressure rise values being exceeded.
It is advantageous to vent the control volume via a flow resistance, in which case a particle filter is connected upstream and the differential pressure between intermediate volume pressure and pressure downstream of the flow resistance is measured, and the degree of soiling of the filter is additionally determined therefrom.
With regard to the device, the core idea of the invention consists in the fact that a control volume is designed between a first valve-rod seal and a second valve-rod seal, and that at least one pressure sensor and/or pressure switch is used to monitor the pressure of this control volume, and this control volume is logic-connected to an evaluation unit or a position regulator in order to determine a leak flow rate. With regard to the device, in this connection it is, of course, highly advantageous to use a flow resistance and filter to vent or empty the control volume.
To determine the degree of soiling of the filter, a further pressure sensor is arranged downstream of the filter, and to determine the degree of soiling the differential pressure between the pressure in the control volume and the pressure downstream of the filter is measured, it being possible, if appropriate, to generate a command and/or a signal to change the filter.
Advantageously, it is now possible to vent or empty the control volume via a switching valve which can be actuated by means of a pressure switch. It is also advantageous for the switching valve to be designed as a pressure-relief valve.
Furthermore, it is now possible for the switching valve to be driven by means of an evaluation unit or a position regulator.
The switching valve may be designed as a directional control valve, in particular as a 2/2 directional control valve.
If pollutants are present in the working pressure medium, the control volume (30) can be vented into a closed or closeable volume.
When suitable leak flow rate limit values are reached, it is, in an advantageous configuration, possible for a command and/or a signal to change the valve-rod seal to be generated automatically. On account of the fact that the state of the seal is in fact determined using remote monitoring, a device of this type can be used particularly advantageously as a process valve which can be monitored remotely.